PURPOSE: To obtain a thermosensor being not affected by electromagnetic induction by shading a part of an emitted ray, which is transformed to a parallel ray, by means of a shading plate that is driven by a change of temperature.

CONSTITUTION: The ray emitted from the optical fiber 1 is transformed into a parallel ray by the lens 2, the parallel ray is focused by the lens 2' and led to the fiber 1'. The shading plate 5 attached to the bimetal 3 is located so that it shades a part of the parallel ray. Since the shading plate 5 moves perpendicularly to the optical axis as the bimetal 3 deforms with temperature, the quantity of light received by the fiber 1' varies. Accordingly temperature can be measured by a change of the light quantity. Because the thermosensor changes the luminous intensity or spectrum, no electric signal is included in the sensor and transmission line. Thus temperature of the object of a superhigh-pressure can exactly be measured.